A terrestrial digital broadcasting system transmits a terrestrial signal through an allocated channel with high power using a transmitter and a repeater. The transmitter and the repeater include a channel filter at end thereof. In general, an analog broadcasting scheme uses a taboo channel to reduce the interference to adjacent channel. For example, if a broadcasting station uses the 9th channel, the other broadcasting station cannot use adjacent channels of the 9th channel which are the 8th channel and the 10th channel in the analog broadcasting scheme. However, a digital broadcasting scheme can use such adjacent channels. That is, the digital broadcasting scheme can effectively utilize limited frequency resources. In order to use the adjacent channels, a channel filter is used at the end of a digital broadcasting transmitter and repeater. The channel filter is a bandpass filter for removing unnecessary frequency components so as to reduce the interference in the adjacent channels.
However, the channel filter has a disadvantage that deteriorates the quality of a transmitting signal because the channel filter has amplitude response characteristics and group delay response characteristics in a non-uniform band. In order to overcome the disadvantage of the channel filter, a pre-equalizer is used.
FIG. 1 is a block diagram illustrating a wireless transmitting block in a VSB transmitter and a repeater according to the related art.
Referring to FIG. 1, the vestigial side band (VSB) transmitter converts TV programs to a digital signal and transfers the digital signal to a transmitting block. Meanwhile, the repeater receives a radio signal from the transmitter and transfers the received radio signal to the transmitting block in order to retransmit the received radio signal. The wireless transmitting blocks of the VSB transmitter and the repeater have the same configuration.
A pre-equalizer 101 is disposed in the front of the VSB modulator 102 to compensate the amplitude distortion and the group delay distortion of the channel filter 106. The pre-equalizer 101 outputs an equalized signal to the VSB modulator 102, and the VSB modulator 102 modulates the equalized signal to a predetermined format.
That is, the VSB modulator 102 outputs a VSB modulated intermediate frequency (IF) digital signal. The VSB modulator 102 outputs the VSB modulated IF digital signal to the digital-to-analog converter 103. The digital-to-analog converter 103 converts the VSB IF digital signal to an analog IF signal. Then, a frequency up converter 104 receives the analog IF signal from the digital-to-analog converter 103 and up-converts the received analog IF signal to an analog radio frequency (RF) signal. A high power amplifier 105 receives the analog RF signal and amplifies the received analog RF signal to the high power analog RF signal. Finally, the high power analog RF signal is transmitted through a transmission antenna after passing through the channel filter 106.
An adaptive pre-equalizer for changing a tap coefficient according to a channel state may be disposed in the front end of the VSB modulator 102 as the pre-equalizer 101. In order to calculate the tap coefficient of the pre-equalizer 101, a signal coupled with the output signal of the channel filter 106 is inputted to the frequency down converter 107.
The frequency down converter 107 converts the coupled analog RF signal to an analog IF signal. The analog-to-digital converter 108 converts the analog IF signal to the digital IF signal. The VSB demodulator 109 receives the digital IF signal and restores a baseband signal from the digital IF signal.
A pre-equalizer filter coefficient generator (not shown) calculates a tap coefficient for the pre-equalizer 101 using the restored baseband digital signal from the VSB demodulator 109 and the input signal of the VSB modulator 102. The pre-equalizer 101 also compensates the amplitude distortion and the phase distortion in a transmission band, which are generated in a path from the VSB modulator 102 to the channel filter 106 as well as compensating the amplitude distortion and group delay distortion in the band of the channel filter 106.
However, the amplitude distortion and the phase distortion are also generated in a transmission band of a feedback path from the channel filter 106 to the VSB demodulator 109. Accordingly, the tap coefficient of the pre-equalizer may have error because the pre-equalizer filter coefficient generator (not shown) uses the output signal of the VSB demodulator 109 to calculate the tap coefficient of the pre-equalizer 101. As a result, the performance of the pre-equalizer is deteriorated due to the error of the tap coefficient.